The long term goals of this project are to identify and characterize new pathways involved in the regulation of mitosis and to identify proteins in mitotic pathways whose functions are altered in transformed cells. Numerous core pathways exist in regulating mitosis. Most core pathways are evolutionarily conserved; however vertebrates have acquired auxiliary pathways involved in fine tuning mitotic processes. While most conserved mitotic regulators are characterized, vertebrate mitotic regulators functioning in auxiliary pathways remain poorly characterized for several reasons. First, they may play subtle roles in contributing to increased chromosome segregation fidelity, but are not essential for the cell division process. Second, proteins may have evolved additional functions in other areas of cell cycle control or cell physiology, making identification as direct mitotic regulators difficult. Finlly, auxiliary pathways may be supplemental in normal cells, but have become essential in cancer cells. Through bioinformatics, we have generated a list of uncharacterized or poorly characterized candidate genes with predicted mitotic functions. Preliminary examination of candidates has allowed identification of GTSE1 (G2 and S phase expressed 1) as a putative novel mitotic regulator in vertebrates. Importantly, GTSE1 has been found to be differentially expressed in several cancers. This proposal aims to: (1) Test whether GTSE1 functions directly in mitosis and examine its importance for transformed vs. non- transformed cells; and (2) characterize the pathways which GTSE1 functions in during mitosis. Through use of the recently developed auxin-inducible degron (AID) system combined with RNAi-mediated depletion and cell cycle stage specific rescue, we will determine if GTSE1 functions during mitosis. To address the role of GTSE1 in cancer, we will determine if GTSE1 is more essential for mitosis in transformed cells by comparing a panel of transformed and non-transformed cell lines for differences in both GTSE1 protein levels and dependence upon GTSE1 for mitotic progression. Finally, mass spectrometry coupled with in vitro and in vivo assays will allow identification of novel GTSE1 interacting proteins and define pathways of function. Accomplishing the goals proposed will not only define the mitotic function of GTSE1, but will further our understanding of the mechanisms regulating mitosis. Additionally, the identification of novel mitotic regulators may lead to individualized cancer therapies targeting pathways required for tumor cells but dispensable for normal cells.